The numbers in brackets below refer to references listed in the Appendix, the teachings of which are hereby incorporated by reference.
There has been a great deal of interest in the successful bonding of metallic sheet to ceramic substrates driven by the several potential applications in the market for medium and high power storage and circuit devices. One known bonding method comprises placing a metal member such as copper in contact with a non-metallic substrate such as alumina, heating in a furnace to a temperature slightly below the melting point of metal (e.g. between approximately 1,065.degree. and 1,080.degree. C. for copper), the heating being performed in a reactive gaseous atmosphere, such as an oxidizing atmosphere, for a sufficient time to create a copper-copper oxide eutectic melt, in the copper/oxygen case, which, upon cooling, bonds the metal to the substrate. Gas flow rates used are approximately two cubic feet per hour of argon-oxygen gas mixture. Approximately one cubic foot per hour of the argon-oxygen gas mixture produces a total oxygen content in the combined gases of approximately 0.04 molar percent [1]. This method was later improved with a reaction atmosphere between 0.01 and 0.5 a.w. % oxygen in nitrogen [2]. In both situations, the big problem was the oxidation and precipitation of copper oxide in the bonded member. To eliminate this problem, it was proposed to heat the copper and ceramic in a vacuum at a pressure no greater than one millibar and a partial oxygen pressure between 0.001 and 0.1 mbars, with the oxygen pressure under 0.005 mbars at the time of cooling [3]. This, however, requires expensive equipment and a control over the oxygen partial pressure.
No literature references have been found regarding the use of lasers to promote bonding of metal to ceramic materials; however, the known literature contains information about heat flow and heat conduction where a laser beam heat source is used. Two models have been previously reported; the hyperbolic model that predicts a severe thermal wavefront at the surface and the parabolic model that predicts a continuous temperature rise followed by diffusion of heat into the medium when the pulsed train is deactivated. In fact, the differences between the hyperbolic and parabolic models become less extreme as the pulse frequency decreases [4-6]. An investigation of the laser weldability of copper employing thin layers of TiO, colloidal graphite and Cr was reported by Daurelio and Ammannati [7]. Their results indicated the importance of employing an absorption coating during the laser processing of copper. Other investigators reported the relationship between the growth kinetics of oxide films and their effect on the absorption of radiation during laser heating [8-10]. Nonuniformity in heating the oxide film at high incident radiation intensities, and significant temperature gradients in the oxide layer are capable of producing cracking of the oxide coating and peeling from themetal surface, which may in turn degrade heat removal from the oxide film in the metal substrate [11]. In the known literature [12] it is stated "in general a suitable ceramic for laser machining must possess low parameters of elasticity and thermal expansion as well as high breaking strength and a high heat and thermal conductivity".
Those concerned with these and other problems recognize the need for an improved method of bonding metal to a non-metal substrate.